Frog lessons: More biodiversity equals less disease

Researchers from the University of Colorado Boulder have been on a frog hunt.

They’ve spent three years scouring hundreds of California ponds in search of amphibians with mangled, grotesque deformities. Sometimes, the frogs have misshapen legs. Sometimes, they have extra sets of legs sprouting where they don’t belong. And sometimes, they have no legs at all.

By cataloging the deformed frogs — and the toads, newts and salamanders that share their homes — the researchers have made an important discovery: more diversity equals fewer deformations.

Ponds that were home to a half-dozen amphibians species were more resistant to the parasitic infections that can cause the deformities than ponds that were home to just one kind of amphibian.

The lessons learned from the study, published in the journal Nature, may be relevant to how biodiversity and disease, including those that attack humans, are linked in much larger ecosystems, such as forest and grasslands. For example, a larger number of mammal species in an area may curb cases of Lyme disease, while a larger number of bird species may slow the spread of West Nile virus.

“How biodiversity affects the risk of infectious diseases, including those of humans and wildlife, has become an increasingly important question,” said Pieter Johnson, an assistant professor in the Department of Ecology and Evolutionary Biology and lead author of the study. “But as it turns out, solidly testing these linkages with realistic experiments has proven very challenging in most systems.”

In all, the CU-Boulder researchers spent three years sampling 345 wetlands and recording deformations caused by parasitic infections in 24,215 amphibians. They also cataloged 17,516 snails, which act as an intermediate host for the parasites. The results showed that ponds with half a dozen amphibian species had a 78 percent reduction in parasite transmission compared to ponds with just one amphibian species. The research team also set up experiments in the lab and outdoors using 40 artificial ponds, each stocked with 60 amphibians and 5,000 parasites.

The reason for the decline in parasitic infections as biodiversity increases is likely related to the fact that ponds add amphibian species in a predictable pattern, with the first species to appear being the most prone to infection and the later species to appear being the least prone. For example, the research team found that in a pond with just one type of amphibian, that amphibian was almost always the Pacific chorus frog, a creature that is able to rapidly reproduce and quickly colonize wetland habitats, but which is also especially vulnerable to infection and parasite-induced deformities.

On the other hand, the California tiger salamander was typically one of the last species to be added to a pond community and also one of the most resistant to parasitic infection. Therefore, in a pond with greater biodiversity, parasites have a higher chance of encountering an amphibian that is resistant to infection, lowering the overall success rate of transmission between infected snails and amphibians.

While the new study has implications beyond parasitic infections in amphibians, it does not mean that an increase in biodiversity always results in a decrease in disease, Johnson cautioned.

Photos

CU-Boulder researchers collect recently metamorphosed amphibians from a pond in the Bay Area of California to check them for malformations. Photo courtesy of Dave Herasimtschuk / Freshwaters Illustrated.